The increasing use of automated manufacturing equipment in various industries has led to a need for more accurate robotic manipulators which can perform a broader range of tasks of more exacting nature. Until recently, prior art robotic manipulators were generally variations of three basic types of designs. One of these designs is the link and pivot design which utilizes a series of pivotally supported segments with an end-effector. An example of a link and pivot robot would be one in which a grasping device or a welding gun or the like is attached to the distal end of the link. A second type of prior art robot has extending links in combination with pivots, wherein the end points of the links translate along the axes of the links relative to one another. A third type of robot design is shown in U.S. Pat. Nos. 3,922,930 and 3,739,923.
In particular, this third type of robot features a plurality of serially-connected rotatable drive shafts to provide two or more axes of pivotable motion at a common point, with such movement being remotely operated. As mentioned above, modern applications for automated manufacturing equipment demand not only greater flexibility but greater accuracy as well. The flexibility and accuracy offered by a previously available industrial robot is dependent upon its programming and upon the physical orientational and positional capability of its moveable parts. Moreover, a robot's accuracy is dependent upon the positioning and orientation capabilities of the end-effector attached to the distal end of a moveable robotic arm.
U.S. Pat. No. 4,068,536, which issued to T. Stackhouse on Jan. 17, 1978, utilizes a robotic wrist section attached to the distal end of a robotic arm including a plurality of serially-connected rotatable drive shafts, as generally described by the above-referenced patents. The Stackhouse patent, however, discloses a unique structural design of its wrist section which increases both the orientational and positional capabilities thereof. The Stackhouse arrangement teaches the use of three serially-connected rotary shafts having axes which intersect at a single point to undergo continuous "rolls" while avoiding the mechanical interference inherent in prior art devices. The Stackhouse manipulator is capable of orienting a part normal to any point on a spherical sector generated by rotating the manipulator through space. This capability of orienting a part normal to any point on the generated spherical sector eliminates "holes" or "voids" in the spacial orientation of the end-effector, thereby increasing the manipulator's flexibility. However, as mentioned above, in addition to increased flexibility, modern day robotic manipulators increasingly must provide substantial accuracy in the movements of their end-effector. While the Stackhouse manipulator provides excellent flexibility, interaction of its gears and shafts permits the entrance of a limited amount of slack or backlash into the movements of the wrist. This slack or backlash is commonly inherent in situations where a plurality of gears interact to affect movement of a remote piece. Required clearances and other tolerances in the gear meshes create such inherent backlash, and this backlash can be quite detrimental to the overall "tightness" or accuracy of the resulting movement. Therefore, despite substantial advances in the flexibility of robotic manipulators, there remain problems of eliminating inherent slack in gearing systems which detract from the accuracy of the end-effectors of such manipulators.